1. Field of the Invention
This invention relates to a method of producing a bonded silicon wafer and more particularly to a method of producing a bonded silicon wafer in which an active layer is thinned by polishing with an oxygen ion implanted layer as a polishing stop layer and also a method of producing a bonded silicon wafer in which polishing stop in thinning is conducted with a high accuracy. In the invention, the term “polishing” includes both chemical polishing (etching) and mechanical polishing. Moreover, the production method of the invention is applicable to both SOI (Silicon On Insulator) wafers and DSB (Direct Silicon Bonding) wafers.
2. Description of the Related Art
The SOI wafer is superior in the isolation between devices, reduction of parasitic capacity between a device and a substrate and possibility of three-dimensional structure to the conventional silicon wafer. Therefore, it is used in LSI with high speed and low power consumption. As a method of producing an SOI wafer is known a bonding method in which two silicon wafers provided with an insulating layer (BOX layer) are bonded to each other and thereafter ground and polished to form an SOI layer (active layer). This bonding method includes a Smart Cut (Registered Trademark) method (JP-A-H09-116125 and JP-A-2000-124092).
Thinning and thickness uniformity of SOI layer (active layer) are desired in the SOI wafer. Thus, a new method has been developed. In this method, after a silicon wafer for active layer having an oxygen ion implanted layer is bonded to a silicon wafer for support layer, the oxygen ion implanted layer is changed to an SiO2 layer by a heat treatment, and grinding and polishing are conducted from the side of silicon wafer for active layer to the SiO2 layer, and thereafter the SiO2 layer is removed to enable thinning and thickness uniformity of SOI layer (active layer) (WO 05/074033 A1). The oxygen ion implanted layer serves as a polishing stop layer.
Recently, the thickness of the insulating layer (BOX layer) is reduced with the miniaturization and lower power consumption of devices and further it is performed to develop DSB wafers by directly bonding a silicon wafer for active layer having no insulating layer (BOX layer). In case of direct bonding without an insulating film, after the general cleaning (SC1), a silicon wafer for active layer is bonded to a silicon wafer for support layer.
However, the production method of the bonded silicon wafer using the polishing stop layer as described in WO 05/074033 A1 has the following problems.
A part (silicon layer) of the wafer for active layer is polished by chemical polishing action with a caustic solution as a polishing solution. When etching is conducted with the caustic solution, the difference of etching rate between silicon and SiO2 is large, but it is very difficult to judge whether the polishing reaches the oxygen ion implanted layer only by such a difference of etching rate. Thus, it is required that the polishing is stopped once to set a polishing time by measuring the thickness of the silicon wafer composite or calculating a polishing rate.
Moreover, as the polishing of the silicon layer proceeds, the oxygen ion implanted layer (layer including SiO2) is at a partly exposed state. At this state, the layer including SiO2, namely the oxygen ion implanted layer is polished by not only the chemical polishing action using the difference of etching rate between silicon and SiO2 but also mechanical polishing action, so that the variation in the thickness of the active layer becomes large. In addition, there is a problem that the oxygen ion implanted layer in the silicon wafer for active layer is over-etched by the mechanical polishing action during polishing of a silicon wafer composite formed by bonding the silicon wafer for active layer to the silicon wafer for support layer.